Constant velocity universal joints are widely used in applications where power is transmitted from one to another of rotational axes coupled to each other at an angle, such as in the power transmission system of automobiles or various types of industrial machines. Ball type constant velocity universal joints use balls as torque transmitting elements. The balls retained with a cage are set in ball tracks of an inner ring and an outer ring. The track centers are equally offset in opposite directions relative to the joint center so as to maintain the balls in a bisecting plane of the input shaft and the output shaft to keep the constant speed.
In a cross groove constant velocity universal joint (“Lobro joint” or “LJ”), for example, which has ball tracks inclined relative to the rotational axis, the ball tracks of the inner ring and outer ring are inclined relative to the rotational axis symmetrically with each other at the same angle so as to satisfy the requirements of a constant velocity universal joint and to achieve flexibility. FIG. 7A and FIG. 7B illustrate the shape of an inner ring or its intermediate product (hereinafter “product”) of an LJ. Reference numeral 12 in the drawing represents ball tracks in which balls will roll when the product is assembled as a constant velocity universal joint. The ball tracks are formed at six locations in the outer circumference of the product 10. These ball tracks 12 are inclined relative to the axial line of the product 10, and adjacent ball tracks 12 are inclined oppositely from each other. Outer circumferential parts 14 other than the ball tracks 12 are generally partially spherical.
Since the LJ inner ring is an irregular shape component, it is generally produced through forging and machining processes. That is, a plurality of divided dies formed with a projection at an inner end to form the ball tracks are circumferentially equally arranged such as to surround a stock, which is then pressed in the axial direction using a pair of punches to make the material extend radially in order to form the ball tracks.
A forging die for forming the ball tracks is, as described for example in Japanese Patent Application Laid-Open Nos. Sho 57-56132 and Sho 62-193938, divided in the circumferential direction in the number of tracks to be provided in the product. The positioning of dies in the circumferential direction was achieved by providing a guide die for guiding the dies in a radial manner and by accommodating the dies in the guide die. The positioning of dies in the radial direction was achieved by providing the dies with a conical back surface and by fitting a ring having a conical surface on its inner circumference with the outer side of the dies.
With these conventional methods described above, the dies slide on the guide die in a radial manner, and a gap needs to be provided between the dies and the guide die. This makes the guide width of the guide die which is for positioning the dies in the circumferential direction larger than the die width dimension, because of which it is difficult to improve the precision in the circumferential direction of the product. Moreover, since the die back surface forms part of the conical surface, the dies have low rigidity against a force applied at an angle, because of which the dies totter due to the pressure exerted during the processing, which makes it difficult to improve the precision of the angle of inclined grooves. Therefore the inclined grooves in the product are not formed with a sufficient precision with respect to the circumferential pitch and angle.
The applicant of the present invention has proposed a forging method and apparatus, as described in Patent Document 1, which can solve these problems described above and which can achieve high precision required in the circumferential pitch and inclination angle of the grooves in the product.    [Patent Document 1] Japanese Patent Application Laid-Open No. 2000-140984